Variable permeability tuning system



Sept. 9, 1941. w. F. SANDS ETAL 2,255,580

I VARIABLE PERMEABILITY TUNING SYSTEM Filed May 17, 1940 PER CENTFwzpuz/vcy zwwmva/v l N N o u FREQUENCY A C Patented Sept. 9, 19412,255,880 VARIABLE PERMEABILITY TUNING SYSTEM William F. Sands, WestCoillngswood, and Paul F. G. Holst, Oaklyn, N. 1., alsignors to RadioCorporation of America, a corporation of Delaware Application May 17,1940, Serial No. 335,708

Claims.

This invention relates to variable permeability tuning systems for radiosignal circuits, and has for its primary object to provide an improvedtuning system of the character referred to,

wherein two or more signal circuits are tunable through predeterminedfrequency ranges in such a manner as to provide substantially a constantfrequency difierence between the resonant frequencies of two of saidcircuits.

In a superheterodyne radio signal receiving system, it is a furtherobject of the invention to provide an improved variable permeability tuming system for causing the oscillator and signal input circuits to trackaccurately one with the other throughout a predetermined frequency rangeand to compensate the tendency for such circuits to depart from thedesired constant frequency difference.

There are several known methods and means for effecting tracking in thetuning of two or more circuits by variable permeability tuning means,some of which are impracticable because of cost or manufacturingdifiiculties, and others of which are adaptable for commercialapplication.

As an improvement over certain of the latter, it has been found that twoor more variable permeability tuned circuits may be caused to tunethrough the same or differing frequency ranges with accurate tracking bythe combination of certain features as will hereinafter be described,

by way of example, in an application to the tuning of a superheterodynereceiving system.

It is also a further object of the present invention to provide animproved tuning system for a superheterodyne receiver which provides forthe alignment of and a desired tracking relation between two tunablecircuits at a plurality of points in the tuning range.

It is a further object or the invention to provide an improvedoscillator tuning-inductance which is adapted for causing alignment ortracking of the oscillator circuit with a second tunable signal circuitat a plurality of points in a predetermined frequency range.

The invention will be further understood, however, from the followingdescription, when com sldered in connection with the accompanyingdrawing, and its scope is pointed out in the appended claims. 1

In the drawing,

Figure 1 is a schematic circuit diagram of a tuning system for asuperheterodyne I receiver emb dy the invention,

stantially full size, of the tuning elements of Fig. 1 arranged inoperative relation to each other,

Figure 3 is a modification of a portion of the tuning arrangement ofFig. 2 on the same scale and in cross section, and

Figure, 4 is a graph showing a curve illustrative of the trackingrelation between two of the circuit systems shown in Fig. 1.

Referring to Fig. l, the radio signal tuning system shown comprises atunable signal input circuit 5 and a tunable oscillator circuit 6 pr0-vided with variable permeability tuning inductance and shunt capacitymeans.

In the signal input circuit, the variable tuning inductance l isprovided with a movable tuning core indicated at 8, while in theoscillator or second circuit 5, the tuning inductance is indicated by atwo part, continuous winding 9 and i0, having a common movable tuningcore H connected with the first tuning core 8, as indicated by thedotted line i2, for unitary tuning control movement.

The circuit 5 includes a shunt tuning capacitor l3 for the inductance land a shunt trimmer capacitor 14, the latter being adjusted to establishthe low frequency end of the tuning range when the core 8 is moved fullywithin the windmg The circuit 5 is connected to ground as indicated atI5 and is connected at its high potential side to any suitable signalsource, such as an antenna is, through suitable coupling means, such asacoupling capacitor I l. l

The second tuned circuit 6 likewise includes shunt capacity means forthe inductance 9l0 comprising two series connected capacitors l8 and IS,the latter being adjustable in a similar manner to the capacitor H, foradjusting the tuning at the low frequency endof the tuning range of thecircuit 6.

The oscillator circuit 6 in the present example is of the Colpitts type,having a cathode connection 20 between the capacitors l8 and I9, andbeing grounded as indicated at 2|, at the low potential side thereof.

The input circuit 5 is connected at its high potential side to a signalinput grid 22 of a combined detector-oscillator tube 23. The cathode 24is connected to ground 25 through a suitable choke coil 26," the cathodeterminal 21 of which is connected with the cathode tap 20 of theoscillator circuit.

The circuit 8 is connected with a second signal Figure 2 is a view, incross section and subgrid 28 in the tube 23 for electronically mixingquency of the R.-F.

the signals from the circuit with oscillationsto produce a predeterminedintermediate irequency in the output anode circuit 29 or thedetector-oscillator tube 23. Feedback for the production of oscillationsis provided in connection with the screen grid 36. which functions as ananode coupled to ground through a bypass capacitor ii in connection witha filter resister 38 in the screen grid supply circuit indicated at 33.The oscillator grid is provided with the usual grid coupling capacitort4 and grid leak 85 to ground.

In connection with the circuit 5 and the signal input grid 22, acoupling capacitor 88 and grid resistor 31 are provided, the latterbeing connected to a bias supply lead 88, with a suitable by-pass toground through a lay-pass capacitor as.

Signals are derived from the detector-oscillator and the output circuit39 through a suitable tuned intermediate frequency coupling transformer80.

Referring now to Fig. 2 along with Fig. 1, the same reference numeralsare used to designate like parts as in the preceding figure, the signalinput circuit inductance being indicated at l and the oscillator tuninginductance being indicated at 9-40 and their respective tuning coresbeing shown at 8 and H. The inductance windings are supported onsuitable insulating cylindrical coil forms 45 and 46, respectively,which are secured in spaced parallel relation to each other to a fixedmounting frame 41.

The cores are connected with operating rods 40 extending into the openends of the coil forms and are adiustably connected with a movable framemember 49 having a body or main portion 50 which passes through guideopenings 5! in the frame 41 to permit unitary movement of the two coresinto and out of the windings. This tuning movement is provided as theframe 50 is moved to the right and left, as viewed in the drawing, undercontrol of a tuning control knob 52 which serves to drive a wrapped cord58 connected at its ends with the frame 50 representing any suitablearrangement for this purpose.

The present preierred arrangement for actuating the movable coreelements is of the type wherein a plurality of. core elements are con--nected with a common actuating means providing for the same length oftravel for all of the core elements and, therefore, substantially thesame coil or winding length in association with each coil, since it isnecessary for maximum tuning range, for the core to move from a positionat the entering end of the winding to a position at which the core isfully entered in the winding. Therefore, since all of the cores mustmove the same distance, the coils are all substantially of the samelength.

This arrangement is satisfactory for a plurality of circuits which aretuned through the same frequency range. However, in case that kc. or ina 3:1 frequency ratio while the Oscillator must be tuned through afrequency range of 1000 to 1950 kc. or in a 2:1 frequency ratio.

Since the frequency of a tuned circuit changes inversely as the squareroot of the inductance, it is apparent that a smaller inductancevariation will be required for the oscillator circuit than for theantenna or input circuit. The inductance becomes a maximum when the ironcore fills the space within the solenoid, as nearly as practicable. Anincrease in the diameter of the oscillator coil. therefore, willdecrease the inductance variation range. In the present example, thediameter of the oscillator coil is increased to a value providingsubstantially exact tracking at the ends of the tuning range.

For this reason, the diameter of the oscillator winding 8l0 is madegreater than the R.-F. winding as shown in Fig. 2. Assuming that thewindings l and 9 are of substantially the same length, as hereinbeforedescribed, for unitary control of the tuning cores, the diameter of theoscillator inductance is made slightly larger than that of the R.-F.inductance; so that the R.-F. oscillator circuits will be in alignmentor provide the desired frequency difference at 550 and 1700 kcs. in thepresent example. For such a broadcast receiver, the oscillator coil formdiameter may be approximately .400 inch, while the coil form diameter ofthe R.-F. winding may be .275 inch approximately, both windings beingapproximately 1% inches long.

While this relation between the diameters of the windings providestracking at the extreme ends of the tuning range, it tends to cause adeparture from a desired tracking relation in the midrange and this iscompensated or corrected by winding one of the inductances, preferablythe oscillator tuning inductance, with a variable pitch winding, thenumber of turns per inch increasing either uniformly or in steps towardthe high frequency end of the winding which is the end at which the coreenters. If in steps, a, plurality of difiering winding pitches may beprovided on one winding.

However, a stepped or variable pitch winding is diflicult to produce andit is, therefore, more one circuit is tunable through a different fre 1quency range, as in the present example, where the circuit 6 is that ofthe oscillator and the circuit 5 is that for the R.-F. input signal,means rangesand with a predetermined tracking or frequency differencerelation.

Assuming, for example, that the tuning system shown is adapted for thepresent broadcast band of 550 to 1500 kc. and that an intermediate ire-450 kc. is desired, it will be seen that circuit must be tuned from 550to 1500 costly than the usual solenoid winding having a uniform windingpitch. It has been found, however, that means may be provided foreffecting a variable pitch or obtaining the efiects of a variable pitchin a tuning inductance of the variable 1 permeability type having amovable core element.

This includes providing a plurality of back wound turns at the highfrequency or entering end of the inductance winding, such as indicatedat H! in Figs. 1 and 2, the portion I0 being a continuation of theportion 9 in the same winding direction. This produces the effect of a.difierent winding pitch in the winding section 9 covered by the section10.

By way of example, an oscillator coil form 46 of .403 inch diameter maybe wound as at 9 for 1% inches approximately, with No. 33enameled wire,.double spaced and equal to 64 turns per inch, and then may be back woundas at I for' spect to the 3.4 tuning core, for example,

inch, so that the R.-1". tuning core enters the-extreme high frequencyend of the R.-l". inductance slightly. in advance of the entry of theoscillator core into the oscillator tuning inductance, this relationbeing shown in the present example and provided by threading the ends ofthe rods 48, as indicated at 55, and fitting the threaded ends withsuitable lock nuts 58 for securing the rods after the desired relativespacing of the cores and coils is provided.

The arrangement is such that one of the cores enters the winding withwhich it is associated in advance of the other core entering itsassociated winding. In the case of the oscillator and R.-F. circuitsassumed herein, by way of example, the R.-F. core enters in advance ofthe oscillator core.

With the arrangement shown and as described, the tracking deviationbetween the oscillator and R.-F. tuning circuits is substantially asindicated by the curve 58 shown in Fig. 3, the crossover points 59 and60 being due to the back-wound turns, and the crossover point 6! beingdue to this arrangement and the fact that the cores enter the windingsin slightly spaced consecutive order as described. As previouslyreferred to, the tracking points 62 and 63 along the zero axis at thelow and high frequency ends are provided by the predetermined relationbetween the diameters of the two windings.

It will be seen that this tracking relation is highly desirable sincethe frequency deviation curve crosses the zero axis at a plurality ofpoints between the high and low frequency ends of the tuning range andthat this tracking relation is obtained by a comparatively simplecircuit and tuning element arrangement involving comparatively low costin production.

The relation between the incremental change in inductance for anincremental change in the position of the tuning core, or the derivativeof the tuning response 'curve for a permeability tunable system has beenfound to be greatest toward the center of the tuning band, and in theband chosen herein, for example, occurs in the region of 1200 kcs.

It is, therefore, desirable to adjust the position of the back-woundturns with respect to the tuning core at this frequency. As indicated inFig. 3, the oscillator winding 9 is provided with the hack-wound turnsIII on a movable short coil form or sleeve 65, whereby the turns Ill maybe moved along the winding 9 and positioned as desired, for maximumcontrol of the tracking response, when the coil form 65 may be cementedor otherwise secured in place by any suitable means.

It will be noted that, in the arrangement of Fig. 3, the winding 9 is oflarger diameter than the antenna or R.-F. inductance as in Fig. 2, as isrequired for producing the required shortening of the oscillator tuningrange, and that the backwound turns ID are, inany case, located at oradthecombination of two iacent to the lower potential or ground end ofthe winding, which is also the high frequency end, since the core entersat that end. The windings 9 and III are serially connected in aidingrelation so that they form one. continuous winding, the winding I0having a fewer number of turns than the winding 9 and being shorter inaxial length.

One method of effecting circuit alignment with this arrangementcomprises adjusting the 'trimmer capacitors I4 and It, as shown in Fig.l, at the low frequency end of the tuning band with the cores fullypenetrating the inductance windings of the coil, thereby providingcircuit alignment at the low frequency end of the tuning range orranges, as the case may be.

The tuning cores are adjusted for maximum response at a frequency atwhich the derivative of the tuning curve has the highest value, such asapproximately 1200 kcs. in the present frequency band underconsideration. The relative position of the windings Ill and S then maybe adjusted for maximum sensitivity, and the winding I 0 is then securedin place on the winding It will be noted that this arrangement has theeffect of changing the pitch of the turns of the winding 8 which liedirectly under the turns of the winding Ill without necessitatingwinding the oscillator coil with a variable pitch or a multiple pitchwinding.

We claim as our invention:

1. In a variable permeability tuning system, the combination of twotuning inductance windings each having a movable tuning core and one ofsaid windings having a plurality of back wound turns adjacent one endfor effecting a variable pitch in said windings thereby to provide apredetermined tracking relation in the tuning response of said windingswith unicontrol tuning movement of said cores.

2. In a variable permeability tuning system,

tuning inductance windings each having a movable tuning core adapted toenter one end thereof in tuning, means for moving said cores in unisonto vary said tuning, and one of said windings having a plurality of backwound turns adjacent said end for effecting a variable pitch in saidwinding, thereby to provide a predetermined tracking relation in atuning response of said winding with unicontrol tuning movement of saidcores.

3. In a variable permeability tuning system, the combination of aplurality of tuning inductance windings each having a movable tuningcore, said cores being interconnected for unitary control of the tuningof said windings, and means for effecting a variable pitch in one ofsaid windings thereby to effect tracking in the tuning response of saidwinding with respect to that of the other of said windings, said meanscomprising a plurality of back-wound turns forming a continuation of and:being located adjacent to one end of said last-named winding,

4. In a variable permeability tuning system,

the combination of a plurality of tuning inductance windings each havinga movable tuning core, said cores being interconnected for unitarycontrol of the tuning of said windings, means for effecting a variablepitch in one of said windings thereby to eifect tracking in the tuningresponse of said winding with respect to that of the other of saidwindings, said means comprising a plurality of back-wound turns forminga continuation of and being located adjacent to one end of saidlast-named winding, and shunt capacitor tuning means for each of saidwindings connected in circuit therewith, said capacitor means beingadjustable for effecting a predetermined tracking relation between thetuning of said windings at one end of the tuning ranges thereof.

5. In a variable permeability tuning system, the combination ofaplurality of tuning inductance windings of substantially the samewinding length, each having a movable tuning core of substantially thesame diameter, said cores being interconnected for unitary control ofthe tuning of said windings through predetermined frequency ranges withthe same length of travel of said cores, and means for effecting avariable pitch in one of said windings thereby to eflect tracking in thetuning response of said winding with respect to that or the other ofsaid windings, said means comprising a plurality of back-wound turnsforming a continuation of and being located adjacent to one end of saidlast-named windings, and fixed shunt capacitor tuning means for each ofsaid windings connected in circuit therewith.

6. In a variable permeability tuning system,

' the combination of a plurality of tuning inductthereby to effecttracking in the tuning response of said winding with respect to that orthe other of said windings, said means comprising a plurality ofback-wound turns forming a continuation of and being located adjacent toone end of said last-named windings, and fixed shunt capacitor tuningmeans for each of said windings connected in circuit therewith, saidcapacitor means being adjustable for effecting a predetermined trackingrelation between the tuning of said windings at one end of the tuningranges thereof. v y

7. In a variable permeability tuning system, the combination of a pairof tuning inductance windings of relatively differing diameters, thelarger diameter winding having a plurality of back-wound turns adjacentone end thereof for effecting a variable winding pitch therein, a pairof movable tuning core members of comminuted ferro-magnetic material onefor each of said windings, means for moving core elements in unison withrespect to said windings, one of said cores being displaced in thedirection or" movement to enter its associated winding in advanceassaeso winding.

8. In a variable permeability tuning system,

vthe combination or a pair 0! tuning inductance windings, one of saidwindings having a plurality of back-wound turns adjacent one end thereoffor efiecting a variable winding pitch therein, a pair of movable tuningcore members of comminuted term-magnetic material one for each of saidwindings, means for moving said core elements in unison with respect tosaid windings, one of said cores being displaced in the direction ofmovement to enter its associated winding in advance of the entry of theother core into its associated winding.

9. In a variable permeability tuning system, the combination of aplurality of tunable signalconveying circuits and means in one of saidcircuits for efiecting a predetermined tracking relation in the tuningof said circuit with at least one other of said circuits, said meanscomprising an inductance winding of the solenoid type having a pluralityof back-wound turns forming a continuation thereof adjacent andsurrounding one end and a movable tuning core of comminutedterm-magnetic material movable into the said end of said winding to varythe tuning thereof.

10. In a variable permeability tuning system,

' the combination of a plurality of tunable signal conveying circuitsand means in one or said circuits for eflectinga predetermined,'trackingrelation in the tuning of said circuit with at least one other of saidcircuits, said means comprising in inductance winding of the solenoidtype having a plurality of back-wound turns forming a continuationthereof adjacent and surrounding one end and a movable tuning core ofcomminuted term-magnetic material movable into the said end of saidwinding to vary the tuning thereof, the diameter of said core elementbeing so related to the diameter and length or the winding that thetuning range of said wniding is predetermined and fixed with relation tothe tuning of others of said circuits.

. F. SS.

PAUL F. G. HOLST.

